Solvent-free conversion of oxiranes to thiiranes with thiourea

Article abstract of DOI:10.1080/10426500490466599

A simple and efficient method for the conversion of various oxiranes to the corresponding thiiranes using thiourea under solvent free conditions is described.

Full text of DOI:10.1080/10426500490466599

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Phosphorus, Sulfur, and Silicon and the Related
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SOLVENT-FREE CONVERSION OF OXIRANES TO
THIIRANES WITH THIOUREA
Ali Reza Kiasat a , Foad Kazemi a & Mehdi Fallah Mehr Jardi a
a Chemistry Department, College of Science, Shahid Charman University , Ahvaz, Iran ,
6137-4-3169
Published online: 16 Aug 2010.
To cite this article: Ali Reza Kiasat , Foad Kazemi & Mehdi Fallah Mehr Jardi (2004) SOLVENT-FREE CONVERSION OF
OXIRANES TO THIIRANES WITH THIOUREA, Phosphorus, Sulfur, and Silicon and the Related Elements, 179:9, 1841-1844, DOI:
10.1080/10426500490466599
To link to this article: http://dx.doi.org/10.1080/10426500490466599
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Phosphorus, Sulfur, and Silicon, 179:1841–1844, 2004
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490466599
SOLVENT-FREE CONVERSION OF OXIRANES
TO THIIRANES WITH THIOUREA
Ali Reza Kiasat, Foad Kazemi, and Mehdi Fallah Mehr Jardi
Chemistry Department, College of Science, Shahid Charman
University, Ahvaz 6137-4-3169, Iran
(Received January 22, 2004)
A simple and efficient method for the conversion of various oxiranes
to the corresponding thiiranes using thiourea under solvent free condi-
tions is described.
Keywords: Oxirane; solvent free; thiirane; thiourea
INTRODUCTION
In organic syntheses and reactions, increasing attention is being fo-
cused on green chemistry, using environmentally benign reagents and
conditions, particularly solvent-free procedures,1 which often lead to
clean, eco-friendly, and highly efficient procedures involving simplified
workups.2 Reactions under dry conditions were originally developed in
the late 1980s3 and offer several advantages.4 The absence of solvent
reduces the risk of hazardous explosion when the reaction takes place
in a closed vessel. Moreover, aprotic dipolar solvents with high boiling
points are expensive and are difficult to remove from reaction mixtures.
As the most interesting class of cyclic sulfides, thiiranes serve as use-
ful precursors for the synthesis of olefins by phosphite- or phosphite-
mediated desulfurizations and other functional group moieties,6 and so
their synthesis is of fundamental interest. A variety of synthetic meth-
ods exist to prepare thiiranes;7−14 one of the most intriguing routes in-
volves the reaction of epoxides with inorganic thiocyanates or thiourea
as sulfurintroducing reagents.11−18
The authors acknowledge the partial support of this work by Shahid Chamran Ahvaz
University Research Council.
Address correspondence to Ali Reza Kiasat, Chemistry Department, College of Sci-
ence, Shahid Charman University, Ahvaz 6137-4-3169, Iran.
1841

1842
A. R. Kiasat et al.
In comparison with ammonium thiocyanate, thiourea is a less-
reactive and more-stable sulfurating agent and its handling is easier.19
Therefore, application of oxiranes as convenient starting materials
and thiourea as sulfurating agent can be considered as a practical
and useful achievement in the synthesis of thiiranes. Conversion of
oxiranes to thiiranes with thiourea was previously reported under
wet conditions or in aqueous ethanol,11 but the reactions suffer from
long reaction times, low yield, and the occurrence of desulfuration of
obtained episulfide to olefin in some cases. Recently, application of
poly (4-vinylpyridine)-Ce(OTf)4,16 tin(IV) mesotetraphenylporphyrin,17
TiO(CF3CO2)2,18 TiCl3(CF3SO3),18 Bi(TFA),19 and RuCl320 as catalysts
for this conversion was reported.
RESULTS AND DISCUSSION
In continuation of our ongoing program to develop synthetic protocols
for the conversion of oxiranes to thiiranes,12−14,21−23 we report here con-
ditions whereby various types of thiiranes can be conveniently synthe-
sized from the corresponding oxiranes under mild nonaqueous reaction
conditions by thiourea under solvent-free conditions (Figure 1).
The substrates used (cyclohexene and styrene oxides, glycidyl phenyl
ether, glycidyl isopropyl ether, and allyl glycidyl ether) were selected
as examples of aliphatic, cyclic, activated, and deactivated epoxides.
With this approach these substrates are converted to the corresponding
thiiranes as exclusive and virtually pure products according to TLC and
1H NMR. The obtained results are summarized in Table I.
In reports where an aqueous solvent has been used, control of pH is
important to obtain high yields of thiiranes without polymerization.5
Our procedure provides good yields of thiiranes in comparatively short
time, without formation of any polymeric by-product.
We reasoned that the mechanism of the reaction involves a SN2-type
nucleophilic attack of the thiourea on the epoxide, Furthermore, if this
reasoning was valid and the reaction rate was second order, then in-
creasing the concentration of each reactant should result in an increase
of the rate of reaction. Thus the reaction was carried out neat with a
FIGURE 1

Thiiranes
1843
TABLE I Conversion of Epoxides to Thiiranesa with Thiourea
Under Solvent-Free Condition
Entry
1
Substrate
Product
Time (min) Yield %b
15
65
2
3
4
15
60
60
84
77
80
5
25
92
aProducts were identified by comparison of their physical and
spectral data with those of authentic samples.
bIsolated yield.
variety of epoxides, and to our satisfaction the neat reactions were fast
and clean, typically affording the episulfides in good yields (Table I).
CONCLUSION
In conclusion, this efficient method can be applied for conversion of
different classes of epoxides carrying activated and deactivated groups
into their corresponding thiiranes. Short times, simple workup, and
mild reaction conditions make this method a useful addition to the
present methodologies.
EXPERIMENTAL
General
Products were characterized by comparison of their physical data with
those of authentic samples. All yields refer to isolated products. TLC
accomplished the purity determination of the substrates and reactions
monitoring on silica gel polygram SILG/UV 254 plates.

1844
A. R. Kiasat et al.
General Procedure for the Conversion of Oxiranes
to Thiiranes
A stirred mixture of epoxide (1 mmol) and thiourea (2 mmol) was heated
at 120◦C with a constant temperature bath for the appropriate time,
as shown in Table I. The progress of the reaction was monitored by
TLC. The mixture was cooled to room temperature, and CCl4 (20 ml)
was added to the mixture and filtered. Evaporation of the solvent under
reduced pressure gave the pure products in 65–92% isolated yields.
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